1. Field of the Invention
The present invention relates to a zoom lens for use in a video camera, electronic still camera or the like using an imaging element, an imaging device and information device having such a zoom lens.
The zoom lens of this invention can be used as an imaging zoom lens in an imaging device such as a silver salt camera, digital still camera, video camera, or digital video camera. The information device of this invention can be used as a digital still camera, portable digital assistant or the like.
2. Description of the Related Art
In recent years, a zoom lens has generally been used in photographing optical systems for use in digital still cameras or the like. A zoom lens including an approximate 50 mm field angle in a focal length range in 35 mm conversion is especially known.
With respect to these zoom lenses, users strongly request downsizing, wide angle, and high-speed auto focusing (hereinafter, referred to as AF).
Various positive lead type zoom lenses including in order from an object side to an image side a first lens group having a positive refractive power, a second lens group having a negative refractive power and a subsequent group are conventionally known (refer to Patent Document 1: Japanese Patent Application Publication No. H03-228008, Patent Document 2: Japanese Patent Publication No. 3716418, Patent Document 3: Japanese Patent Publication No. 3397686, Patent Document 4: Japanese Patent Publication No. 4401451 and Patent Document 5: Japanese Patent Application Publication No. 2010-175954). Such a zoom lens can easily increase a magnification ratio and downsize an entire length with a positive lead type lens.
Each of the zoom lenses described in Patent Documents 1-5 includes an inner focus type lens. The zoom lens described in Patent Document 1 performs focusing by the movement of the second lens group. Each of the zoom lenses described in Patent Documents 2-5 performs focusing by the movement of the third lens group.
In the zoom lens described in Patent Document 1, which performs focusing by the movement of the second lens group, the sizes of a motor and an actuator are likely to be increased, and the maximum diameter of a lens barrel is also likely to be increased because of a large weight of the second lens group.
Such a zoom lens has a problem in high-speed AF and quiescence in animation photographing because the second lens group has a large weight.
Each of the zoom lenses described in Patent Documents 2-5 performs focusing by the third lens group having a negative refractive power.
However, it is difficult to significantly reduce a weight of the focusing group of the zoom lens in each of Patent Documents 2-4. In the zoom lens described in Patent Document 5, the third lens group for focusing is constituted by one negative lens, so that the weight of the focusing group is reduced; thus, high-speed AF and a small diameter lens barrel can be accomplished. However, it is considered that the performance of the third lens group should be further improved and the zoom lens should be further downsized.
In recent years in which a zoom lens has mainly been used, high-speed AF is required as described above. It is necessary for a zoom lens to reduce an entire length of lenses (a distance from a lens surface on the most object side to an image side) when using a zoom lens in order to downsize the zoom lens.
It is also necessary to downsize a focusing lens for increasing an AF speed.
Moreover, considering the application of a zoom lens to a high-end digital camera, it is necessary to have resolution corresponding to an imaging element having at least 10 million pixels over the entire zooming area.
A zoom lens which is suitable for a high magnification ratio includes in order from the object side to the image side a first lens group having a positive refractive power, a second lens group having a negative refractive power, a third lens group having a negative refractive power, a fourth lens group having a positive refractive power and a fifth lens group having a positive refractive power.
Such a conventional zoom lens including a five-group constitution of positive, negative, negative, positive, positive is described in Patent Document 6 (Japanese Patent Application Publication H10-48518) and Patent Document 7 (Japanese Patent Application Publication 2000-28923).
The zoom lens described in Patent Document 6 includes a five-group constitution of positive, negative, negative, positive, positive in a specific example, and performs focusing by moving a third lens group as a focusing group. However, the third lens group is large and heavy because the third lens group is a cemented lens made of negative and positive lenses. For this reason, a load for moving the focusing group is increased and a size of a motor or the like for driving the focusing group and a time for focusing are also increased.
The zoom lens described in Patent Document 7 includes a five-group constitution of positive, negative, negative, positive, positive in Embodiment 4, and performs focusing by moving the third lens group as a focusing group. However, the third lens group is large and heavy because the third lens group includes three lenses of negative, positive and negative. For this reason, a load for moving the focusing group is increased and a size of a motor or the like for driving the focusing group and a time for focusing are also increased similar to Patent Document 6.